1. Field of the Invention
The present invention relates to handover in a broadband wireless access communication system, and more particularly to an apparatus and method for a soft handover in a communication system employing an orthogonal frequency division multiple access scheme.
2. Description of the Related Art
Fourth generation (4G) communication systems (the next generation communication system) are being designed to provide users with services having various Qualities of Service (QoS) supporting a transmission speed of about 100 Mbps. Current third generation (3G) communication systems support a transmission speed of about 384 kbps in a relatively unfavorable outdoor channel environment, and support a maximum transmission speed of 2 Mbps in a relatively favorable indoor channel environment.
Wireless Local Area Network (LAN) system and wireless Metropolitan Area Network (MAN) system generally support transmission speeds of 20 to 50 30 Mbps. In current 4G communication systems, research is being conducted to develop a new type of communication system for ensuring the mobility and QoS in the wireless LAN and MAN systems supporting relatively high transmission speeds in order to support a high speed service to be provided by the 4G communication system.
Since the wireless MAN system has a wide service coverage area and supports a high transmission speed, it is suitable for supporting high speed communication services. However, the wireless MAN system reflects a single cell structure, without providing for the mobility of the user, i.e., a Subscriber Station (SS); the wireless MAN system does not provide for a handover to accommodate high speed movement of the SS.
Research is being conducted to develop an apparatus and scenario for supporting a handover according to high speed movement of the SS. A representative system for reflecting movement of the SS is the IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system. In this document, an SS having mobility is referred to as a ‘Mobile Subscriber Station (MSS)’.
FIG. 1 is a block diagram schematically illustrating the cell structure of a general IEEE 802.16e communication system.
The IEEE 802.16e communication system has a multi-cell structure, that is, has a cell 100 and a cell 150. In addition, the IEEE 802.16e communication system includes a Base Station (BS) 110 controlling the cell 100, a BS 140 controlling the cell 150, and a plurality of MSSs 111, 113, 130, 151 and 153. The transmission/reception of signals between the BSs 110 and 140 and the MSSs 111, 113, 130, 151 and 153 is executed according to the Orthogonal Frequency Division Multiplexing (OFDM) and the Orthogonal Frequency Division Multiple Access (OFDMA) schemes.
From among the MSSs 111, 113, 130, 151 and 153, the MSS 130 is located in a cell boundary area, i.e., handover area, between the cell 100 and the cell 150. Accordingly, only when handover for the MSS 130 is supported, is it possible to support the mobility of the MSS 130.
The wireless MAN system is a broadband wireless access (BWA) communication system, which has a wider service coverage and supports a higher transmission speed than the wireless LAN system. The IEEE 802.16e communication system employs the OFDM scheme and the OFDMA scheme to enable a physical channel of the wireless MAN system to support a broadband transmission network.
As described above, the IEEE 802.16e system supports handover for an MSS, but supports only a hard handover scheme. According to the hard handover scheme, when a hard handover is performed, the MSS terminates all connections to a serving BS currently providing service before establishing a new connection to another BS, i.e., to a target BS, from which the MSS desires to receive new service.
In the IEEE 802.16e communication system, when the intensity, i.e., the carrier-to-interference-and-noise ratio (CINR), of a signal received from a current serving BS decreases to such a degree that it is impossible to maintain communication with the current serving BS, the MSS performs a handover to a neighbor BS (i.e., target BS) in response to a request from the MSS or the current serving BS.
However, while the MSS is performing a handover operation to the target base station in the IEEE 802.16e communication system, if the CINR of a signal received from the target base station decreases to such a degree that it is impossible to receive a desired service from the target base station, the MSS can change its connection to the serving base station. For example, signal shadowing occurs due to obstructions on the wireless channel. When an MSS passes through a cell boundary area, this is, when the MSS is located in a handover area, a phenomenon occurs where the CINR of a signal received from the target base station becomes higher and then lower than that of a signal received from the serving base station occurs. If it is determined that a handover is initialized when the intensity of a signal received from the target base station becomes equal to that of a signal received from the serving base station, a handover operation may occur repeatedly while the MSS is passing through the cell boundary area. Such a phenomenon is called a ‘ping-pong effect’. When the ping-pong effect occurs, handover signaling greatly increases, so that the probability of handover failure also increases.
FIG. 2 is a schematic graph for explaining a ping-pong effect occurring in the IEEE 802.16e communication system.
FIG. 2 shows a ping-pong effect occurring according to the performance of the conventional hard handover when an MSS moves from a first base station (BS 1) to a second base station (BS 2). To be specific, FIG. 2 shows a graph for illustrating the intensities of signals received from the first and second BSs to the MSS when the MSS is located in a handover area which is a service coverage overlapped by the first and second BSs. In the following description, it is assumed that the first BS is a serving BS and the second BS is a target BS.
Referring to FIG. 2, when the MSS moves from the serving BS (BS 1) to the target BS (BS 2), a handover is executed at three time points in total, i.e., at time points ‘A1’, ‘A2’ and ‘A3’. This is because it is assumed that the normal IEEE 802.16e communication system performs a hard handover, and that the hard handover is performed at a time point at which the CINR of a signal received from the target BS becomes equal to that of a signal received from the serving BS.
As described above, the occurrence of the ping-pong effect increases the signaling load of the system, which increases the probability of handover failure and also deteriorates performance of the entire system.
To avoid the ping-pong effect which is problematic of the hard handover, a handover parameter called ‘Hysteresis margin’ may be used. In other words, while the MSS moves from the serving BS to the target BS, a handover is performed only when the intensity of a signal received from the target BS is greater than that of the signal received from the serving BS by the Hysteresis margin. When the Hysteresis margin is used, unnecessary handover operations caused by the ping-pong effect are prevented.
However, when the Hysteresis margin is used, a handover is performed not in the handover area, but at a location near the target BS, i.e., at a location near the target BS from a cell boundary. Therefore, as compared with the case where the hysteresis margin is not used, the intensity of a signal received from the serving BS at the cell boundary may be very poor.
In FIG. 2, when a Hysteresis margin is set as ‘H’ and the Hysteresis margin ‘H’ is used, the MSS performs a handover only once at time point ‘B’. However, it can be confirmed that the intensity of a signal received from the serving BS when the Hysteresis margin is used, is smaller than that when the Hysteresis margin is not used. Thus, when the Hysteresis margin is used, the intensity of a signal received from the serving BS is poor, so that the connection between the MSS and the serving BS may be cut off before the MSS completes handover to the target BS.
To solve the problem of the hard handover as described above, a soft handover scheme has been proposed. The soft handover scheme is a communication technique, wherein the MSS establishes a connection to the target BS before ending a connection to the serving BS so that the MSS performs communication with two BSs (i.e. the serving BS and target BS) at the same time in a predetermined cell boundary area, i.e. in a handover area.
When the soft handover is performed in a downlink, the serving BS and the target BS transmit the same data to one MSS through wireless channels occupying the same frequency band at the same point in time. In addition, when the soft handover is performed in an uplink, both the serving BS and target BS receive a signal transmitted from the MSS. Therefore, when the soft handover scheme is employed, it is possible to prevent both the ping-pong effect which is a problematic of the hard handover and the phenomenon of decreasing signal intensity. In addition, when the soft handover is employed, the MSS is allocated with wireless channels simultaneously from the two BSs in a downlink, so that the CINR of a received signal is improved. In addition, since the two BSs simultaneously receive a signal transmitted from one MSS in an uplink, it is possible to improve the quality of the uplink by applying a macro diversity scheme to two signals received in the serving BS and the target BS.
However, although the soft handover has the above-mentioned advantage, a difficulty lies in applying the soft handover as it is without changing the current standardized subchannel allocation scheme in the normal IEEE 802.16e communication system. That is, to provide the soft handover, two neighbor BSs, i.e. a serving BS and a target BS, must allocate the same subchannel including the same sub-carriers at the same time. Herein, the subchannel represents a channel including at least one sub-carrier, and sub-carriers included in the subchannel may or may not be neighbor to each other in the frequency domain.
FIG. 3 is a diagram for schematically illustrating the frame structure of a normal IEEE 802.16e communication system.
First, the frame includes a Downlink frame (“DL frame”) and an Uplink frame (“UL frame”). The downlink frame includes a preamble area, a broadcasting control area and a data transmission area. The broadcasting control area includes a downlink MAP (“DL-MAP”) area and an uplink MAP (“UL-MAP”) area. The data transmission area may be classified into a partial-usage-of-subchannels (“PUSC”) area and a full-usage-of-subchannels (“FUSC”). The “PUSC” subchannel and the “FUSC” subchannel may be distinguished by time division in the same frame. In addition, the uplink frame includes an FUSC area and a PUSC area. FIG. 3 illustrates a frame structure for sub-carrier allocation of a sector using a downlink PUSC, a downlink FUSC and an uplink PUSC.
A synchronization signal (e.g., a preamble sequence) for acquiring synchronization between a transmitter and a receiver, i.e., between a BS and an MSS is transmitted through the preamble area. A DL-MAP message and a UL-MAP message are transmitted through the DL-MAP area and the UL-MAP area, respectively. Herein, information elements (IEs) included in the DL-MAP message and the UL-MAP message have no direct relation with the present invention, so description thereof will be omitted.
The PUSC area represents data burst including subchannels based on a PUSC scheme, and the FUSC area represents data burst areas including subchannels based on a FUSC scheme.
According to the PUSC scheme, each sector allocates and uses only partial subchannels of the whole subchannels. When the PUSC scheme is employed, the frequency reuse factor becomes larger than “1”. Therefore, PUSC areas different from each other are allocated to the sectors of two neighbor cells so as to avoid mutual interference between sectors. However, it is problematic when two BSs allocate an MSS located at a cell boundary with a PUSC area having the same sub-carrier.
According to the FUSC scheme, all sectors of all cells allocate and use the whole subchannels. When the FUSC scheme is employed, the frequency reuse factor becomes “1”. However, when the FUSC scheme is employed, although all sectors can use all of the subchannels, a distinct set of sub-carriers configuring a subchannel must be configured according to each sector. That is, it is necessary to design the FUSC so as to minimize a probability that corresponding sub-carriers between subchannels are overlapped, i.e. a hit probability. It is necessary to allocate the same subchannel having the same sub-carriers to two sectors to provide the soft handover, but it is impossible, using the current FUSC scheme, to provide such subchannel allocation. In other words, the current communication system has been proposed for the subchannel configuration schemes supporting the FUSC and PUSC schemes, but does not provide a distinct subchannel configuration scheme for supporting the handover scheme.
In spite of the merit of soft handover as described above, it is nearly impossible, under the existing subchannel structure, to provide soft handover that simultaneously uses the same channels. Therefore, it is necessary to develop a scheme capable of providing soft handover in a broadband wireless access communication system. Accordingly, a new data transmission method and system for the conventional soft handover is required.